Transparent hydrophobic mixed oxide coatings and methods

ABSTRACT

A hydrophobic coating and a method for applying such a coating to a surface of a substrate. The method includes applying a coating composition to the surface and heating the coated surface at a cure temperature from about 300° C. to about 600° C. for a time from about 2 hours to about 48 hours. The coating composition is applied to the surface by an application method selected from the group consisting of flowing, dipping, and spraying. The coating composition comprises a yttrium compound, an additive selected from the group consisting of a cerium compound and a dispersion of yttrium oxide nanoparticles, a water-soluble polymer, and a solvent solution of de-ionized water and a water-soluble alcohol.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/681,140, filed on Aug. 18, 2017 and entitled “TRANSPARENT HYDROPHOBICMIXED OXIDE COATINGS AND METHODS,” which is a continuation-in-part ofU.S. patent application Ser. No. 15/242,372, filed on Aug. 19, 2016 andentitled “TRANSPARENT HYDROPHOBIC MIXED OXIDE COATINGS AND METHODS,”which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates generally to transparent, hydrophobic coatingsand, more particularly, to a hydrophobic coating comprising rare-earthoxides and methods of applying such a coating to a surface of asubstrate, such as glass.

BACKGROUND

Controlling the wetting properties of surfaces has been the subject ofscientific investigation. Most existing hydrophobic surfaces rely on lowsurface energy polymers, such as fluoroalkylsilane, or patternedroughness at low length scales. Both strategies have significantdrawbacks. For example, fluorinated polymers lack resistance to abrasionand are easily degraded by ultraviolet light. Similarly, high roughnesscoatings are often fragile and poorly suited for harsh environments. Inaddition, these coatings often rely on complex manufacturing techniquesthat are not easily scalable.

Aircraft, automotive, and other transparency applications provideadditional challenges. For these applications, a hydrophobic coatingshould maintain high hardness, optical transparency, and resistance toattack by acids and bases. Moreover, because these applications ofteninvolve thermally and chemically tempered glass, which rapidly loses itsstrength at temperatures of 500° C. or higher, it is desirable to have acoating formulation having a cure temperature within the safe limits ofglass substrate melting or de-tempering.

It should be appreciated that there is a need for a scalable method ofapplying an improved hydrophobic coating having environmentally robusthydrophobicity. The improved coating should be robust to environmentaldegradation, mechanical abrasion, and repeated stress, while exhibitinginherently low surface energy without additional surface patterning. Forapplications involving, for example, transparent, tempered glass, thecoating should maintain hardness, optical transparency, and resistanceto attack by acids and bases, while having a cure temperature within thesafe limits of glass substrate melting or de-tempering. The presentinvention fulfills these needs and provides further related advantages.

BRIEF SUMMARY OF THE INVENTION

The present invention is embodied in a hydrophobic coating and a methodof applying a hydrophobic coating to a surface of a substrate. In oneembodiment, the method includes applying a coating composition to thesurface and heating the coated surface at a cure temperature from about300° C. to about 600° C. for a time from about 2 hours to about 48hours. The coating composition is applied to the surface by anapplication method selected from the group consisting of flowing,dipping, and spraying. The coating composition comprises a yttriumcompound, an additive selected from the group consisting of a ceriumcompound and a dispersion of yttrium oxide nanoparticles, awater-soluble polymer, and a solvent solution of de-ionized water and awater-soluble alcohol. Each feature or concept is independent, but canbe combined with any other feature or concept disclosed in thisapplication.

In one embodiment, the substrate is glass. In another embodiment, theglass is tempered, and may be both thermally and chemically tempered. Ina further embodiment, the glass may be transparent. In yet anotherembodiment, the substrate is transparent tempered glass. Each feature orconcept is independent, but can be combined with any other feature orconcept disclosed in this application.

In one embodiment, the cure temperature is from about 450° C. to about500° C. In another embodiment, the time may be from about 12 hours toabout 24 hours. In a further embodiment, the cure temperature is about450° C. and the time is about 24 hours. In a more detailed embodiment,the method further comprises the step of allowing the coatingcomposition on the surface of the substrate to dry before heating. In analternative embodiment, the method comprises the step of drying thecoating composition on the surface of the substrate before heating. Inyet another embodiment, the heating step may comprise ramping from astart temperature to the cure temperature at one or more ramp rates.Each feature or concept is independent, but can be combined with anyother feature or concept disclosed in this application.

In one embodiment, the yttrium compound is selected from the groupconsisting of yttrium acetate, yttrium carbonate, yttrium chloride,yttrium fluoride, yttrium hydroxide, yttrium metal, yttrium nitrate,yttrium oxalate, and yttrium sulfate. In a further embodiment, theyttrium compound is yttrium acetate. Each feature or concept isindependent, but can be combined with any other feature or conceptdisclosed in this application.

In one embodiment, the additive is a cerium compound. In an additionalembodiment, the cerium compound is from about 18% to about 32% by weightof the cerium compound and yttrium acetate. In a further embodiment, thecerium compound is about 26% by weight of the cerium compound andyttrium acetate. In another embodiment, the cerium compound is fromabout 0.3% to about 0.6% by weight of the coating composition. In afurther embodiment, the cerium compound is from about 0.4% to about 0.5%by weight of the coating composition. In one embodiment, the ceriumcompound is water-soluble. In another embodiment, the cerium compound isselected from the group consisting of cerium bromide, cerium chloride,and cerium nitrate. In a further embodiment, the cerium compound issparingly water-soluble. In yet another embodiment, the cerium compoundis selected from the group consisting of cerium acetate and ceriumsulfate. Each feature or concept is independent, but can be combinedwith any other feature or concept disclosed in this application.

In one embodiment, the additive is the dispersion of yttrium oxidenanoparticles. In an additional embodiment, the dispersion of yttriumoxide nanoparticles is from about 0.1% to about 5% by weight of thecoating composition. In a further embodiment, the dispersion of yttriumoxide nanoparticles is from about 0.5% to about 1% by weight of thecoating composition. In one embodiment, the cure temperature is fromabout 300° C. to about 500° C. In a further embodiment, the curetemperature is from about 300° C. to about 400° C. In one embodiment,the cure time may be from about 2 hours to about 5 hours. In a furtherembodiment, the cure time may be from about 2 hours to about 4 hours. Ina further preferred embodiment, the cure temperature may be about 300°C. and the time may be about 2 hours. Each feature or concept isindependent, but can be combined with any other feature or conceptdisclosed in this application.

In one embodiment, the water-soluble polymer is selected from the groupconsisting of poly(n-vinylpyrrolidone), poly(vinylamine) hydrochloride,polymethacrylamide, polyvinyl alcohol, polyacrylamide, poly(ethyleneoxide-b-propylene oxide), poly(methacrylic acid), poly(ethylene oxide),poly(n-iso-propylacrylamide), and poly(2-vinylpyridine). In anotherembodiment, the water-soluble polymer is polyvinyl alcohol. In yetanother embodiment, the water-soluble polymer is from about 1% to about10% by weight of the coating composition. In a further embodiment, thewater-soluble polymer is from about 1% to about 5% by weight of thecoating composition. Each feature or concept is independent, but can becombined with any other feature or concept disclosed in thisapplication.

In one embodiment, the water-soluble alcohol is selected from the groupconsisting of isopropyl alcohol, methanol, ethanol, propanol, andbutanol. In another embodiment, the water-soluble alcohol is isopropylalcohol. In a further embodiment, the de-ionized water and water-solublealcohol are present in the solvent solution in a ratio of about 2:1.Each feature or concept is independent, but can be combined with anyother feature or concept disclosed in this application.

A more detailed example is embodied in a method of applying ahydrophobic coating to a glass surface. In one embodiment, the methodincludes applying a coating composition to the surface and heating thecoated surface at a cure temperature from about 450° C. to about 600° C.for a time from about 12 hours to about 48 hours. The coatingcomposition is applied to the surface by an application method selectedfrom the group consisting of flowing, dipping, and spraying. The coatingcomposition comprises a yttrium compound, a cerium compound, awater-soluble polymer, and a solvent solution of de-ionized water and awater-soluble alcohol. The cerium compound is from about 0.3% to about0.6% by weight of the coating composition, the water-soluble polymer isfrom about 1% to about 5% by weight of the coating composition; and thede-ionized water and water-soluble alcohol are present in the solventsolution in a ratio of about 2:1. Each feature or concept isindependent, but can be combined with any other feature or conceptdisclosed in this application.

In other embodiments, the yttrium compound is selected from the groupconsisting of yttrium acetate, yttrium carbonate, yttrium chloride,yttrium fluoride, yttrium hydroxide, yttrium metal, yttrium nitrate,yttrium oxalate, and yttrium sulfate; the cerium compound is selectedfrom the group consisting of cerium acetate, cerium bromide, ceriumchloride, cerium nitrate, and cerium sulfate; the water-soluble polymeris selected from the group consisting of poly(n-vinylpyrrolidone),poly(vinylamine) hydrochloride, polymethacrylamide, polyvinyl alcohol,polyacrylamide, poly(ethylene oxide-b-propylene oxide), poly(methacrylicacid), poly(ethylene oxide), poly(n-iso-propylacrylamide), andpoly(2-vinylpyridine); and the water-soluble alcohol is selected fromthe group consisting of isopropyl alcohol, methanol, ethanol, propanol,and butanol. Each feature or concept is independent, but can be combinedwith any other feature or concept disclosed in this application.

In further embodiments, the yttrium compound is yttrium acetate, thecerium compound is cerium chloride, the water-soluble polymer ispolyvinyl alcohol, the water-soluble alcohol is isopropyl alcohol, andthe cure temperature is about 450° C. and the time is about 24 hours.Each feature or concept is independent, but can be combined with anyother feature or concept disclosed in this application.

Another more detailed example is embodied in a method of applying ahydrophobic coating to a glass surface. In one embodiment, the methodincludes applying a coating composition to the surface and heating thecoated surface at a cure temperature from about 300° C. to about 400° C.for a time from about 2 hours to about 4 hours. The coating compositionis applied to the surface by an application method selected from thegroup consisting of flowing, dipping, and spraying. The coatingcomposition comprises a yttrium compound, a dispersion of yttrium oxidenanoparticles, a water-soluble polymer, and a solvent solution ofde-ionized water and a water-soluble alcohol. The dispersion of yttriumoxide nanoparticles is about 0.5% to about 1% by weight of the coatingcomposition, the water-soluble polymer is from about 1% to about 5% byweight of the coating composition; and the de-ionized water andwater-soluble alcohol are present in the solvent solution in a ratio ofabout 2:1. Each feature or concept is independent, but can be combinedwith any other feature or concept disclosed in this application.

In other embodiments, the yttrium compound is selected from the groupconsisting of yttrium acetate, yttrium carbonate, yttrium chloride,yttrium fluoride, yttrium hydroxide, yttrium metal, yttrium nitrate,yttrium oxalate, and yttrium sulfate; the water-soluble polymer isselected from the group consisting of poly(n-vinylpyrrolidone),poly(vinylamine) hydrochloride, polymethacrylamide, polyvinyl alcohol,polyacrylamide, poly(ethylene oxide-b-propylene oxide), poly(methacrylicacid), poly(ethylene oxide), poly(n-iso-propylacrylamide), andpoly(2-vinylpyridine); and the water-soluble alcohol is selected fromthe group consisting of isopropyl alcohol, methanol, ethanol, propanol,and butanol. Each feature or concept is independent, but can be combinedwith any other feature or concept disclosed in this application.

In further embodiments, the yttrium compound is yttrium acetate, thewater-soluble polymer is polyvinyl alcohol, the water-soluble alcohol isisopropyl alcohol, and the cure temperature is about 300° C. and thetime is about 2 hours. Each feature or concept is independent, but canbe combined with any other feature or concept disclosed in thisapplication.

Other features and advantages of the invention should become apparentfrom the following description of the preferred embodiments, taken inconjunction with the accompanying drawings, which illustrate, by way ofexample, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are flow diagrams showing a method of applying a hydrophobiccoating in accordance with some embodiments.

FIGS. 2A and 2B are graphs showing coating thickness as a function ofreagent molarity and water-soluble polymer content in accordance withone embodiment.

FIG. 3 is an illustration of a nanoparticle “seeding” process forreducing the cure temperature in accordance with one embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference now to FIGS. 1A-1C of the illustrative drawings, there isshown methods of applying a hydrophobic coating to a surface of asubstrate in accordance with embodiments of the invention. In oneembodiment, the method includes the step 110 of applying a coatingcomposition to the surface and a step 130 of heating the coated surface.In certain embodiments, the coated surface can be heated at a curetemperature from about 450° C. to about 600° C. for a time from about 8hours to around 48 hours. In other embodiments, as will be described ingreater detail below, a cure temperature of about 300° C. to about 500°C. for a time from about 2 hours to 5 hours can be achieved. The coatingcomposition is applied by an application method selected from the groupconsisting of flowing, dipping, and spraying. The coating compositioncomprises a yttrium compound; an additive selected from the groupconsisting of a cerium compound and a dispersion of yttrium oxidenanoparticles; a water-soluble polymer; and a solvent solution ofde-ionized water and a water-soluble alcohol. Each feature or concept isindependent, but can be combined with any other feature or conceptdisclosed in this application.

In some embodiments, the resulting hydrophobic coating will exhibitwater-contact angles greater than about 90°, greater than about 95°,greater than about 100°, or greater than about 105°. The hydrophobiccoating will have a thickness of over about 50 nm, over about 75 nm,over about 100 nm, over about 125 nm, over about 150 nm, over about 200nm, over about 225 nm, or over about 250 nm. In addition, thehydrophobic coating will be robust to environmental degradation,mechanical abrasion, and repeated stress. For example, in someembodiments, the hydrophobic coating will exhibit high hardness, opticaltransparency, and resistance to attack by acids and bases.

Another advantage of this method is that the cure temperature is withinthe safe limits of glass substrate melting and, in some embodiments,de-tempering. Accordingly, this method is appropriate for applying ahydrophobic coating to a surface of, for example, a glass substrate,which may be thermally tempered, chemically tempered, or both.

Many of these beneficial features result, in part, from the combinationof a yttrium compound and an additive of a cerium compound or adispersion of yttrium oxide nanoparticles.

Yttrium, by itself, exhibits high hardness, optical transparency, andresistance to attack by acids and bases. However, yttrium displays onlymoderate hydrophobicity, with a maximum water contact angle of about85°, even when subjected to high temperature extended cure conditions.In addition, yttrium has high melting and crystallization temperatures,which generally exceed the safe limits of glass substrate melting orde-tempering.

Cerium crystallizes at a temperature of around 200° C. to around 400°C., which is within the safe limits of glass substrate melting orde-tempering. However, like yttrium, cerium displays only moderatehydrophobicity, with a maximum water contact angle of 85°, even whensubjected to high temperature extended cure conditions. In addition,cerium, by itself, exhibits a reddish color that is undesirable fortransparency applications.

Surprisingly, a coating composition comprising a combination of ayttrium compound and a cerium compound will result in a mixed system,which has yttrium's preferred optical qualities and cerium's reducedcrystallization temperature. The addition of the cerium compound intothe yttrium lattice promotes atomic mobility and a drive towardcrystallization at lower temperatures. This combination furtherincreases hydrophobicity, with water contact angles greater than about90°, greater than about 95°, greater than about 100°, or about 105°.

Similar results are attained by a coating composition comprising acombination of a yttrium compound and a dispersion of yttrium oxidenanoparticles, which, in one embodiment, possess the same crystalstructure of the cured hydrophobic coating. With reference to FIG. 3,these yttrium oxide nanoparticles promote nucleation and crystal growthof the hydrophobic phase of interest and further suppress the necessarycure temperature. As the coating composition is heated, the yttriumoxide nanoparticles act as “seeds” for the developing yttrium oxidecrystals. In other words, with the dispersion of yttrium oxidenanoparticles, yttrium and oxygen ions in the solution can easily findsites on the nanoparticles and create further layers of hydrophobiccrystalline material. Without the nanoparticle seeds, the dissolved ionswould face a nucleation barrier that can only be overcome through theaddition of thermal energy and increased cure temperature. Thus, thedispersion of yttrium oxide nanoparticles effectively lowers the barrierfor nucleation, allowing crystallization to occur at reducedtemperatures compared to pure yttrium. In certain embodiments, a coatingcomposition comprising a combination of yttrium compound and adispersion of yttrium oxide nanoparticles can achieve significantlylower cure temperatures and cure times than would typically be expected.For example, in various embodiments, the cure temperature to crystalizethe coating composition may be between approximately 300° C. to 500° C.and the cure time is between approximately 2 to 5 hours. In a moreparticular embodiment, the cure temperature is between approximately300° C. and 400° C. and the cure time is between approximately 2 to 3hours. In an even more particular embodiment, the cure temperature isapproximately 300° C. and the cure time is approximately 2 hours. Eachfeature or concept is independent, but can be combined with any otherfeature or concept disclosed in this application.

Accordingly, in one embodiment, the coating composition comprises ayttrium compound and an additive of a cerium compound or a dispersion ofyttrium oxide nanoparticles. In another embodiment, the coatingcomposition comprises a yttrium compound and an additive of both acerium compound and a dispersion of yttrium oxide nanoparticles.

The table below provides chemical formulas for the yttrium-basedchemical reagents available for sol-gel synthesis. In one embodiment,the yttrium is selected from the group consisting of yttrium acetate,yttrium carbonate, yttrium chloride, yttrium fluoride, yttriumhydroxide, yttrium metal, yttrium nitrate, yttrium oxalate, and yttriumsulfate. In a preferred embodiment, the yttrium is yttrium acetate.

Yttrium Compound Formula Yttrium Acetate Y(C₂H₃O₂)₃•H₂O YttriumCarbonate Y₂(CO₃)₃•H₂O Yttrium Chloride YCl₃•(H₂O)₆ Yttrium Fluoride YF₃Yttrium Hydroxide Y(OH)₃•H₂O Yttrium Metal Y Yttrium Nitrate Y(NO₃)₃•H₂OYttrium Oxalate Y₂(C₂O₄)₃•H₂O Yttrium Sulfate Y₂(SO₄)₃•(H₂O)₈

In some embodiments, the coating composition comprises an additive of acerium compound. In one embodiment, the cerium compound is from about18% to about 32% by weight of the cerium compound and yttrium acetate.In another embodiment, the amount of cerium compound is about 26% byweight of the cerium compound and yttrium acetate. In a furtherembodiment, the cerium compound is from about 0.3% to about 0.6% byweight of the coating composition. In an additional embodiment, thecerium compound is from about 0.4% to about 0.5% by weight of thecoating composition. In one embodiment, the cerium compound iswater-soluble. Examples of water-soluble cerium compounds include ceriumbromide, cerium chloride, and cerium nitrate. In another embodiment, thecerium compound is sparingly water-soluble. Examples of sparinglywater-soluble cerium compounds include cerium acetate and ceriumsulfate. Each feature or concept is independent, but can be combinedwith any other feature or concept disclosed in this application.

In other embodiments, the coating composition comprises an additive of adispersion of yttrium oxide nanoparticles. The dispersion of yttriumoxide nanoparticles is preferably compatible with the coatingcomposition and can therefore be added at high levels withoutprecipitation. In one embodiment, the dispersion of yttrium oxidenanoparticles is from about 0.1% to about 5% by weight of the coatingcomposition. In a preferred embodiment, the dispersion of yttrium oxidenanoparticles is from about 0.5% to about 1% by weight of the coatingcomposition. Each feature or concept is independent, but can be combinedwith any other feature or concept disclosed in this application.

A preferred embodiment of the coating composition further comprises awater-soluble polymer. This water-soluble polymer component acts toincrease the thickness of the final hydrophobic coating. The hydrophobicnature of the coating composition without the water-soluble polymermakes it resistant to generating high thickness. With reference to FIG.2A, the final thickness of the resulting coating, without the additionof a water-soluble polymer, is limited to less than about 30 nm, whichis too thin for robust performance in, for example, glass aircraftwindows. With reference to FIG. 2B, the addition of a water-solublepolymer to the coating composition increases the final coating thicknessto over about 50 nm, over about 75 nm, over about 100 nm, over about 125nm, over about 150 nm, over about 200 nm, over about 225 nm, or overabout 250 nm. Each feature or concept is independent, but can becombined with any other feature or concept disclosed in thisapplication.

In one embodiment, the water-soluble polymer is selected from the groupconsisting of poly(n-vinylpyrrolidone), poly(vinylamine) hydrochloride,polymethacrylamide, polyvinyl alcohol, polyacrylamide, poly(ethyleneoxide-b-propylene oxide), poly(methacrylic acid), poly(ethylene oxide),poly(n-iso-propylacrylamide), and poly(2-vinylpyridine). In a furtherembodiment, the water-soluble polymer is polyvinyl alcohol. In oneembodiment, the water-soluble polymer is from about 1% to about 10% byweight of the coating composition. In yet another embodiment, thewater-soluble polymer is from about 1% to about 5% by weight of thecoating composition. Each feature or concept is independent, but can becombined with any other feature or concept disclosed in thisapplication.

A preferred embodiment of the coating composition further comprises asolvent solution of de-ionized water and a water-soluble alcohol. In oneembodiment, the water-soluble alcohol is selected from the groupconsisting of isopropyl alcohol, methanol, ethanol, propanol, andbutanol. The table below provides the chemical formulas and the watersolubility levels of some water-soluble alcohols, but any otherwater-soluble alcohol may be used. In another preferred embodiment, thede-ionized water and water-soluble alcohol are present in the solventsolution in a ratio of about 2:1. Each feature or concept isindependent, but can be combined with any other feature or conceptdisclosed in this application.

Compound Formula Solubility In Water Isopropyl Alcohol C₃H₈O MiscibleMethanol CH₃OH Miscible Ethanol CH₃CH₂OH Miscible Propanol CH₃(CH₂)₂OHMiscible Butanol CH₃(CH₂)₃OH 9 g/100 mL

With reference to FIG. 1C, in one embodiment, the method furthercomprises the step 100 of preparing the coating composition. In oneembodiment, the preparing step comprises dissolving a yttrium compound,a water-soluble polymer, and an additive selected from the groupconsisting of a cerium compound and a dispersion of yttrium oxide, in asolvent solution of de-ionized water and a water-soluble alcohol.

In one embodiment, the method comprises the step 110 of applying thecoating composition to the surface by an application method selectedfrom the group consisting of flowing, dipping, and spraying. Theselection of the appropriate method, or combination of methods, iscommonly understood by one of ordinary skill in the art. For example, aflow or spray coating may be appropriate for large parts or complexshapes, or when two different coatings are required. Dip coating may beappropriate, for example, where an entire part is to be coated.

With reference again to FIGS. 1B and 1C, in one embodiment, the methodfurther comprises the step 120 of allowing the coating composition onthe surface of the substrate to dry before heating. In an alternativeembodiment, method comprises the step of drying the coating compositionon the surface of the substrate before heating. In either case, thecoating composition can be allowed to dry for about 1 hour, about 2hours, about 3 hours, or until the coating composition is in the “greenstate.”

In one embodiment, the method comprises the step 130 of heating thecoated surface at a cure temperature from about 450° C. to about 600° C.for a time from about 8 hours to about 48 hours. In one embodiment, thetime is from about 12 hours to about 24 hours. In a preferredembodiment, the cure temperature is about 450° C. and the time is about24 hours.

In another embodiment, the method comprises the step 130 of heating thecoated surface at a cure temperature from about 300° C. to about 500° C.for a time from about 2 hours to about 5 hours. In a more particularembodiment, the cure temperature is between approximately 300° C. to400° C. and the cure time is between approximately 2 to 3 hours. In apreferred embodiment, the cure temperature is about 300° C. and the timeis about 2 hours.

In a further embodiment, the heating step 130 comprises ramping thetemperature from a start temperature to the cure temperature at one ormore ramp rates. For example, a ramp rate can be chosen to allow forslow out-gassing of carbonaceous byproducts without bubble formation ordevelopment of coating hazing. Once most of the compounds are removed,the ramp rate can be increased until the cure temperature is reached.

Another example is embodied in a method of applying a hydrophobiccoating to a glass surface. In one embodiment, the method includesapplying a coating composition to the surface and heating the coatedsurface at a cure temperature from about 300° C. to about 600° C. for atime from about 2 hours to about 48 hours. In a more particularembodiment, the cure temperature is from about 450° C. to about 600° C.and the cure time is from about 12 hours to about 48 hours. The coatingcomposition is applied to the surface by an application method selectedfrom the group consisting of flowing, dipping, and spraying. The coatingcomposition comprises a yttrium compound, a cerium compound, awater-soluble polymer, and a solvent solution of de-ionized water and awater-soluble alcohol. The cerium compound is from about 0.3% to about0.6% weight of the coating composition, the water-soluble polymer isfrom about 1% to about 5% by weight of the coating composition; and thede-ionized water and water-soluble alcohol are present in the solventsolution in a ratio of about 2:1.

In one embodiment, the yttrium compound is selected from the groupconsisting of yttrium acetate, yttrium carbonate, yttrium chloride,yttrium fluoride, yttrium hydroxide, yttrium metal, yttrium nitrate,yttrium oxalate, and yttrium sulfate. In one embodiment, the ceriumcompound is selected from the group consisting of cerium acetate, ceriumbromide, cerium chloride, cerium nitrate, and cerium sulfate. In afurther embodiment, the water-soluble polymer is selected from the groupconsisting of poly(n-vinylpyrrolidone), poly(vinylamine) hydrochloride,polymethacrylamide, polyvinyl alcohol, polyacrylamide, poly(ethyleneoxide-b-propylene oxide), poly(methacrylic acid), poly(ethylene oxide),poly(n-iso-propylacrylamide), and poly(2-vinylpyridine). In yet anotherembodiment, the water-soluble alcohol is selected from the groupconsisting of isopropyl alcohol, methanol, ethanol, propanol, andbutanol.

In one embodiment, the yttrium compound is yttrium acetate. In anadditional embodiment, the cerium compound is cerium chloride. In afurther embodiment, the water-soluble polymer is polyvinyl alcohol. Inanother embodiment, the water-soluble alcohol is isopropyl alcohol. Inyet another embodiment, the cure temperature is about 450° C. and thetime is about 24 hours.

Another example is embodied in a method of applying a hydrophobiccoating to a glass surface. In one embodiment, the method includesapplying a coating composition to the surface and heating the coatedsurface at a cure temperature from about 300° C. to about 500° C. for atime from about 2 hours to about 5 hours. The coating composition isapplied to the surface by an application method selected from the groupconsisting of flowing, dipping, and spraying. The coating compositioncomprises a yttrium compound, a dispersion of yttrium oxidenanoparticles, a water-soluble polymer, and a solvent solution ofde-ionized water and a water-soluble alcohol. The dispersion of yttriumoxide nanoparticles is from about 0.5% to about 1% by weight of thecoating composition, the water-soluble polymer is from about 1% to about5% by weight of the coating composition; and the de-ionized water andwater-soluble alcohol are present in the solvent solution in a ratio ofabout 2:1.

In one embodiment, the yttrium compound is selected from the groupconsisting of yttrium acetate, yttrium carbonate, yttrium chloride,yttrium fluoride, yttrium hydroxide, yttrium metal, yttrium nitrate,yttrium oxalate, and yttrium sulfate. In an additional embodiment, thewater-soluble polymer is selected from the group consisting ofpoly(n-vinylpyrrolidone), poly(vinylamine) hydrochloride,polymethacrylamide, polyvinyl alcohol, polyacrylamide, poly(ethyleneoxide-b-propylene oxide), poly(methacrylic acid), poly(ethylene oxide),poly(n-iso-propylacrylamide), and poly(2-vinylpyridine). In a furtherembodiment, the water-soluble alcohol is selected from the groupconsisting of isopropyl alcohol, methanol, ethanol, propanol, andbutanol.

In one embodiment, the yttrium compound is yttrium acetate. In anotherembodiment, the water-soluble polymer is polyvinyl alcohol. In a furtherembodiment, the water-soluble alcohol is isopropyl alcohol. In anadditional embodiment, the cure temperature is about 300° C. and thetime is about 2 hours.

It should be appreciated from the foregoing description that the presentinvention provides a scalable method of applying a hydrophobic coatingthat exhibits environmentally robust hydrophobicity. Coatings producedby these methods are hydrophobic; optically transparent; and resistantto environmental degradation, mechanical abrasion, repeated stress, andattack by acids and bases. In addition, the coatings are thick enoughfor robust performance and the cure temperature is within the safelimits of glass substrate melting and, in some embodiments,de-tempering. For all of these reasons, the methods described herein,and the resulting coatings, are ideal for aircraft and automotivetransparency applications.

Specific methods, devices, and materials are described, although anymethods and materials similar or equivalent to those described can beused in the practice or testing of the present embodiment. Unlessdefined otherwise, all technical and scientific terms used herein havethe same meanings as commonly understood by one of ordinary skill in theart to which this embodiment belongs.

As used herein, the term “water-soluble” means the compound isinfinitely soluble in water, very soluble in water, freely soluble inwater, or soluble in water, as these terms are commonly understood. Amaterial is generally considered “very soluble” if about 1 gram ofmaterial requires about 1 milliliter or less of solute to dissolve. Amaterial is generally considered “freely soluble” if about 1 gram ofmaterial requires about 1 milliliter to about 10 milliliters of soluteto dissolve. A material is generally considered “soluble” if about 1gram of material requires about 10 milliliters to 30 milliliters ofsolute to dissolve. A material is generally considered “sparinglysoluble” if about 1 gram of material requires about 30 milliliters toabout 100 milliliters of solute to dissolve.

Without further elaboration, it is believed that one skilled in the art,using the proceeding description, can make and use the present inventionto the fullest extent. Other objectives, features, and advantages of thepresent embodiments will become apparent from the following specificexamples. The specific examples, while indicating specific embodiments,are provided by way of illustration only. Accordingly, the presentinvention also includes those various changes and modifications withinthe spirit and scope of the invention that may become apparent to thoseskilled in the art from this detailed description. The followingexamples are illustrative only, and are not limiting of the disclosurein any way whatsoever. Each feature or concept described in each exampleis independent, and can be combined with any other feature or conceptdisclosed in this application.

Example 1

A glass surface was coated with about 5 grams of yttrium acetatedissolved in 200 mL of a 1:2 co-solvent mixture of isopropyl alcohol andde-ionized water. The coated glass substrate was allowed to dry forabout 2 hours before being thermally treated at about 500° C. for about12 hours in ambient atmosphere.

There is a consensus in the scientific literature that yttrium oxidecrystallization proceeds very slowly at temperatures less than 550° C.,which is higher than the cure temperature used in this example andbeyond the material limits (melting and de-tempering points) of theglass substrate. X-ray diffraction measurements of the coated glassshowed peaks consistent with the hydrophobic phase of yttrium oxide. Thehigh peak broadness suggested that, as expected, the yttrium oxidecoating was partially amorphous, requiring higher temperatures or longerdurations for full cure.

The resulting coating thickness did not typically exceed 20 nm with thisprocess. With reference to FIG. 2A, further increases in coatingsolution molarity, up to the maximum reagent solubility, did not producean appreciable increase in final coating thickness. This limitation inthickness is thought to be due to the inherent hydrophobicity of thecoating interfering with film nucleation and growth at the surface.

The performance of coatings in this example was somewhat variable. Someregions of coated glass showed hydrophobicity, with water contact angleslarger than 60°, while other regions showed lower water contact anglesmore consistent with the glass surface. Considering this, and that thecoating thickness is less than the root mean square (RMS) roughness ofthe glass substrate, complete coverage of the surface had not beenachieved.

Example 2

About 5 grams of yttrium acetate was dissolved in 300 mL of a 1:2co-solvent mixture of isopropyl alcohol and de-ionized water and coatingcomposition was doped with about 1% polyvinyl alcohol (PVA) by weight ofthe coating composition. The PVA readily dissolved in the coatingcomposition and acted to increase its viscosity. The PVA furtherimproved wetting conditions at the surface, which promoted a thickercoating front and increased final wet coating thickness. The PVA-basedmatrix allowed for higher rare earth atom content at the surface andconsiderably thicker coatings. After about 2 hours of dry time, thecoated substrates were cured at about 500° C. for about 12 hours in anambient atmosphere. The resulting thickness, determined by surfaceprofilometry, was over 50 nm, far exceeding the 20 nm limit of Example1.

Example 3

A coating composition was prepared as in Example 2, except PVA contentin the coating composition was increased to about 2.5% by weight of thecoating composition. After about 2 hours of dry time, the coatedsubstrates were cured at about 500° C. for about 12 hours in an ambientatmosphere. With reference to FIG. 2B, this process resulted in coatingthicknesses greater than about 150 nm.

Example 4

A coating composition was prepared as in Example 2, except PVA contentin the coating composition was increased to about 5% by weight of thecoating composition. After about 2 hours of dry time, the coatingsubstrates were cured at about 500° C. for about 12 hours in an ambientatmosphere. With reference again to FIG. 2B, this process resulted incoating thicknesses of about 250 nm.

Example 5

A coating composition was prepared as in Example 2 and doped with ceriumchloride in an amount ranging from about 0.00% to about 1% by weight ofthe coating composition. After about 2 hours of dry time, the coatingsubstrates were cured at about 450° C. to about 500° C. for about 8hours to about 48 hours in an ambient atmosphere.

The table below illustrates the effect the cerium compound had on thewater contact angle when the coating was heated at a cure temperature ofabout 450° C. for a time of about 12 hours. As is shown, the contactangle changed from 36°, with no cerium, to 78°, with from about 0.4% toabout 0.5% cerium. This is due to the low temperature generation ofcerium oxide crystallites, which act as nucleation sites for yttriumoxide. Therefore, crystallization of the hydrophobic yttrium oxide phaseis promoted and the necessary cure temperature is lowered.

% Cerium By Weight % Cerium Y₂O₃:Ce Water Contact Angle 0.00%  0% 36°0.02%  1% 44° 0.07%  3% 41° 0.11%  5% 59° 0.16%  7% 62° 0.23% 10% 60°0.27% 12% 64° 0.32% 14% 64° 0.37% 16% 77° 0.41% 18% 71° 0.46% 20% 78°

The table below shows the same effect, although enhanced, for coatingscured at about 475° C. for a time of about 12 hours. These achievedwater contact angles exceeding 80°.

% Cerium By Weight % Cerium Y₂O₃:Ce Water Contact Angle 0.23% 10% 51°0.27% 12% 57° 0.32% 14% 64° 0.37% 16% 71° 0.41% 18% 78° 0.46% 20% 85°

Example 6

The coating composition from Example 3 was doped with cerium chloride inan amount from about 0.2% to about 0.7% by weight of the coatingcomposition. After about 2 hours of dry time, cure temperature andduration were selected to maximize the crystallinity of the hydrophobicceramic coating, thereby boosting water contact angle, while minimizingpotential degradation of the tempered glass substrate.

The tables below show the relationship between water contact angle andcure temperature and time. As is evident, modest cure temperatures canbe used with longer duration. The maximum hydrophobicity and watercontact angle, measuring about 105°, was achieved with a cerium contentfrom about 0.4% to about 0.5% by weight of the coating composition, anda cure temperature of about 450° C. for a time of about 24 hours. Thiscorresponded to a coating composition comprising about 3.69 grams ofyttrium acetate and about 1.31 grams of cerium chloride dissolved in a2:1 co-solvent solution of de-ionized water to isopropyl alcohol withabout 2.5% PVA by weight of the coating composition. This coatingcomposition exhibited high density in the hydrophobic crystal phase,high crystallinity, and a surface morphology with an appropriate degreeof RMS roughness. Longer cure times did not result in any furtherincrease in the observed water contact angle.

450° C. for 24 Hours % Cerium Angle 0.23% 85° 0.27% 85° 0.32% 95° 0.37%95° 0.41% 95° 0.46% 105°  0.50% 95° 0.55% 95° 0.60% 85° 0.64% 85°

450° C. for 8 Hours % Cerium Angle 0.23% 60° 0.27% 64° 0.32% 64° 0.37%77° 0.41% 71° 0.46% 78° 0.50% 64° 0.55% 71° 0.60% 64° 0.64% 78°

450° C. for 16 Hours % Cerium Angle 0.23% 85° 0.27% 77° 0.32% 85° 0.37%85° 0.41% 85° 0.46% 71° 0.50% 71° 0.55% 85° 0.60% 71° 0.64% 71°

The performance of water contact angle and rain shedding was alsoperformed. The coated glass produced water contact angles greater thanabout 100° and minimal optical distortion due to impinging rain.

Although the addition of a cerium compound to the coating compositionallowed for a feasible coating process for glass, it did introduce areduction in mechanical hardness. The mixed oxide Y2O3:Ce coatingsdisplayed greater hardness and scratch resistance than glass, but highcerium content softened the yttrium oxide coating. The followingalternative strategy was considered to achieve closer to the maximumhardness inherent in pure yttrium oxide.

Example 7

The coating composition from Example 2 was doped with a water-basedcolloidal suspension of yttrium oxide nanoparticles in an amount fromabout 0% to about 1% by weight. The colloidal dispersion of yttriumoxide nanoparticles were introduced to substitute for the ceriumcompound in the coating composition. The colloid is compatible with theexisting coating composition and can be added to high levels withoutprecipitation. The resulting water contact angle increased from about71°, with no yttrium oxide nanocrystals, to about 90°, with the additionabout 0.5% of the colloidal suspension; and up to about 99° with about1% of the colloidal suspension.

With reference to FIG. 3, these yttrium oxide nanoparticles promotenucleation and crystal growth of the hydrophobic phase of interest andfurther suppress the necessary cure temperature. As the coatingcomposition is heated, the yttrium oxide nanoparticles act as “seeds”for the developing yttrium oxide crystals. In other words, with thedispersion of yttrium oxide nanoparticles, yttrium and oxygen ions inthe solution can easily find sites on the nanoparticles and createfurther layers of hydrophobic crystalline material. Without thenanoparticle seeds, the dissolved ions would face a nucleation barrierthat can only be overcome through the addition of thermal energy andincreased cure temperature. Thus, the dispersion of yttrium oxidenanoparticles effectively lowers the barrier for nucleation, allowingcrystallization to occur at reduced temperatures compared to with pureyttrium.

Using this strategy, cerium chloride can be replaced with the yttriumoxide colloid to partial or full extent. The hydrophobic oxide crystalphase will encounter “seed” sites on the colloid crystallites withappropriate lattice parameters and electronic environment.

Example 8

A water-based colloidal suspension of yttrium oxide nanoparticles in anamount from about 0% to about 1% by weight was added to the coatingcomposition from Example 2. The colloidal dispersion of yttrium oxidenanoparticles served to substitute for the cerium compound in thecoating composition. The colloid is compatible with the existing coatingcomposition and can be added to high levels without precipitation.Substrates were coated by spray and flow methods before a thermal cureat 300° C. for 2 hours. The resulting water contact angle was over 90°,with the addition about 0.67% of the colloidal suspension; and nearly100° with about 1% of the colloidal suspension. The 300° C. curetemperature necessary to crystallize the coating with the nanoparticlesis greatly reduced from 450° C., the temperature required to fullycrystallize yttrium oxide with cerium addition. As such, in certainembodiments, the nanoparticle nucleation method may be a preferabletechnique to suppress crystallization temperature.

The invention has been described in detail with reference only to thepresently preferred embodiments. Persons skilled in the art willappreciate that various modifications can be made without departing fromthe invention. Accordingly, the invention is defined only by thefollowing claims.

The invention claimed is:
 1. A method of applying a hydrophobic coatingto a surface of a substrate, the method comprising the following steps:applying a coating composition to the surface by an application methodselected from the group consisting of flowing, dipping, and spraying;and heating the coated surface at a cure temperature from about 300° C.to about 500° C. for a time from about 2 hours to about 48 hours; andwherein the coating composition comprises: a yttrium compound, adispersion of yttrium oxide nanoparticles, a water-soluble polymer, anda solvent solution of de-ionized water and a water-soluble alcohol. 2.The method of claim 1, wherein the substrate is glass.
 3. The method ofclaim 1, wherein the cure temperature is from about 450° C. to about500° C.
 4. The method of claim 1, wherein the time is from about 12hours to about 24 hours.
 5. The method of claim 1, further comprisingthe step of drying the coating composition on the surface of thesubstrate before heating.
 6. The method of claim 1, wherein the heatingstep comprises ramping from a start temperature to the cure temperatureat one or more ramp rates.
 7. The method of claim 1, wherein the yttriumcompound is selected from the group consisting of yttrium acetate,yttrium carbonate, yttrium chloride, yttrium fluoride, yttriumhydroxide, yttrium metal, yttrium nitrate, yttrium oxalate, and yttriumsulfate.
 8. The method of claim 7, wherein the yttrium compound isyttrium acetate.
 9. The method of claim 1, wherein the dispersion ofyttrium oxide nanoparticles is in an amount ranging from about 0.1% toabout 5% by weight of the coating composition.
 10. The method of claim9, wherein the amount of the dispersion of yttrium oxide nanoparticlesis from about 0.5% to about 1% by weight of the coating composition. 11.The method of claim 1, wherein the water-soluble polymer is selectedfrom the group consisting of poly(n-vinylpyrrolidone), poly(vinylamine)hydrochloride, polymethacrylamide, polyvinyl alcohol, polyacrylamide,poly(ethylene oxide-b-propylene oxide), poly(methacrylic acid),poly(ethylene oxide), poly(n-iso-propylacrylamide), andpoly(2-vinylpyridine).
 12. The method of claim 11, wherein thewater-soluble polymer is polyvinyl alcohol.
 13. The method of claim 11,wherein the amount of the water-soluble polymer is from about 1% toabout 5% by weight of the coating composition.
 14. The method of claim1, wherein the water-soluble alcohol is selected from the groupconsisting of isopropyl alcohol, methanol, ethanol, propanol, andbutanol.
 15. The method of claim 1, wherein the water-soluble alcohol isisopropyl alcohol.
 16. The method of claim 1, wherein de-ionized waterand water-soluble alcohol are present in the solvent solution in a ratioof about 2:1.